Search for: All records

Abstract Recent investigations have pointed to physical entanglements that greatly outnumber chemical crosslinks as key sources of energy dissipation and low friction in hydrogel networks. Slide-ring gels are an emerging class of hydrogels described by their mobile crosslinks, which are formed by rings topologically constrained to slide along linear polymer chains within the network. These materials have enjoyed decades of study by polymer chemists but have been underexplored by the tribology community. In this work, we synthesized a pseudo-rotaxane crosslinker from poly(ethylene glycol) diacrylate (PEG-diacrylate) andα-cyclodextrin-acrylate followed by hydrogel networks by connecting the sliding crosslinks with polyacrylamide chains. The mechanical and tribological properties of slide-ring hydrogels were investigated using a custom-built microtribometer. Slide-ring hydrogels exhibit unique behavior compared to conventional covalently crosslinked polyacrylamide hydrogels and offer a vast design space for future investigations. Graphical Abstract 

Molecular architectures known as bottlebrush polymers provide unique opportunities to tune the structure and properties of soft materials with applications ranging from rubbers to thin films and composites. This review addresses recent developments and future opportunities in the field with an emphasis on materials science enabled by contemporary bottlebrush chemistry. 

Poly(acrylamide- co -acrylic acid) (P(AAm- co -AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm- co -AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0–12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength ( I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient ( µ ), which decreased with increasing AA concentration. P(AAm- co -AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity ( µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) ( µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm- co -AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface. 

Abstract Conjugated polyelectrolytes (CPEs) exhibit a strong interplay between ionic and electronic properties, enabling tunable photophysical properties and charge transport dynamics. Polyelectrolyte complexation represents a versatile self‐assembly strategy to control the properties of CPEs by forming dense phases with varying optoelectronic and mechanical characteristics. This study focuses on ionically assembled complexes comprising oppositely charged self‐doped CPE (CPE‐K) and bottlebrush polyelectrolyte (BPE). It is demonstrated that subtle adjustments in the composition of CPE‐K:BPE blends enables tuning of photophysical and viscoelastic properties. It is observed that increasing the CPE‐K:BPE monomeric ratio from 1:1 to 1:3 in the initial solution for complexation induces a significant bathochromic shift in the maximum photoluminescence intensity of the dense phase, from 1.8 to 1.4 eV. Additionally, a higher BPE content enhances the softness and adhesion of the solid complex, while maintaining yield‐stress behavior and cyclability of the dense phase. The ability to electrochemically and statically dope the CPE‐K–BPE complex, effectively modulating its charge transport and optoelectronic properties is also demonstrated. This work underscores the potential of these complex‐fluid phases for developing soft, adhesive, and elastic mixed ionic‐electronic conductors with tunable properties for functional applications and 3D‐printing. 

Abstract Direct laser writing (DLW) has been shown to render 3D polymeric optical components, including lenses, beam expanders, and mirrors, with submicrometer precision. However, these printed structures are limited to the refractive index and dispersive properties of the photopolymer. Here, we present the subsurface controllable refractive index via beam exposure (SCRIBE) method, a lithographic approach that enables the tuning of the refractive index over a range of greater than 0.3 by performing DLW inside photoresist-filled nanoporous silicon and silica scaffolds. Adjusting the laser exposure during printing enables 3D submicron control of the polymer infilling and thus the refractive index and chromatic dispersion. Combining SCRIBE’s unprecedented index range and 3D writing accuracy has realized the world’s smallest (15 µm diameter) spherical Luneburg lens operating at visible wavelengths. SCRIBE’s ability to tune the chromatic dispersion alongside the refractive index was leveraged to render achromatic doublets in a single printing step, eliminating the need for multiple photoresins and writing sequences. SCRIBE also has the potential to form multicomponent optics by cascading optical elements within a scaffold. As a demonstration, stacked focusing structures that generate photonic nanojets were fabricated inside porous silicon. Finally, an all-pass ring resonator was coupled to a subsurface 3D waveguide. The measured quality factor of 4600 at 1550 nm suggests the possibility of compact photonic systems with optical interconnects that traverse multiple planes. SCRIBE is uniquely suited for constructing such photonic integrated circuits due to its ability to integrate multiple optical components, including lenses and waveguides, without additional printed supports.